1. Field of the Invention
This invention relates to an apparatus for separating a luminance signal and chrominance signal of a composite video signal.
2. Description of the Prior Art
A conventional luminance signal and chrominance signal separating apparatus is shown in FIG. 13 and includes a composite video signal input terminal 1300, a first delay circuit 1301 for delaying an input signal by one line, a second delay circuit 1302 for delaying an output of the first delay circuit further by one line, a correlation detector 1303 for detecting a correlation of the input signal and the output signal of the first delay circuit 1301 and a correlation of the output signal of the first delay circuit 1301 and an output signal of the second delay signal circuit 1302, a first comb filter 1304 for receiving the input composite video signal and the output signal of the first delay circuit 1301, a second comb filter 1305 for receiving the output signal of the first delay signal circuit 1301 and the output signal of the second delay signal circuit 1302, a band-pass filter 1306 for filtering a frequency band of a color subcarrier in the output signal of the first delay circuit 1301, a selection circuit 1307 for selecting one of the first comb filter 1304, second comb filter 1305 and band-pass filter 1306 in response to the output signal of the correlation circuit 1303 to output an output signal of the filter thus selected, a high-pass filter 1308 for removing the low frequency component from the output signal of the selected filter, a subtractor 1309 for subtracting an output signal of the high-pass filter 1308 from the output signal of the first delay circuit 1301, a chrominance signal output terminal 1310 for outputting the output signal of the high-pass filter 1308, and a luminance signal output terminal 1311 for outputting an output signal of the subtractor 1309.
The luminance signal and chrominance signal separating apparatus as arranged above is applied at its input terminal 1300 with an NTSC signal. Here, suppose that this input signal is expressed as S1, the output signal of the first delay circuit 1301 is expressed as S2 and the output signal of the second delay circuit 1302 is expressed as S3. The first comb filter 1304 subtracts S1 from S2 to output C1 including the chrominance signal and low frequency luminance signal. The second comb filter 1305 subtracts S2 from S3 to output a signal C2 including the chrominance signal and low frequency luminance signal. The band-pass filter 1306 outputs a signal C3 having the frequency band in the vicinity of the color subcarrier.
The correlation detector detects the correlation of S2 and S1 and that of S2 and S3 to output a signal indicating the optimum filter to the selection circuit. The detection of the correlations and the selection of the optimum filter are carried out, for example, as below.
The absolute values of respective high frequency components of S1, S2 and S3 are compared, and in the case where the amplitude difference between S2 and S1 is small, judgment is made so that S1 and S2 are highly correlated and the first comb filter 1304 is considered the optimum filter. On the other hand, in the case where the amplitude difference between S2 and S3 is small, judgment is made so that the second comb filter 1305 is considered the optimum filter. Also, in either case, if the amplitude difference exceeds a predetermined threshold, judgment is made so that there is no correlation of S2 with both S1 and S3 and the band-pass filter 1306 is considered the optimum filter. The selection circuit 1307 selects the optimum filter in response to the output signal of the correlation circuit 1303, and the low frequency component of an output signal of the selected filter 1308 is removed through the high-pass filter to obtain a chrominance signal to be outputted from the chrominance signal output terminal 1310. In addition, the luminance signal is obtained by subtracting the chrominance signal from the output signal of the first delay circuit 1301 to be outputted from the luminance signal output terminal 1311.
With the conventional luminance signal and chrominance signal separating apparatus as structured above, however, if an oblique-stripe luminance signal having a high frequency band is inputted, it is judged that there is no correlation between upper and lower lines, thus causing the band-pass filter 1306 to be selected as the optimum filter. However, the application of the band-pass filter 1306 for such a signal may cause a problem in that the high frequency component of the luminance signal is lost to dim the picture or to be crossed into the chrominance signal to cause a cross color to be generated.
In order to solve these problems, a second example of a conventional luminance signal and chrominance signal separating apparatus has been and is shown in FIG. 14 proposed. This apparatus includes a first delay circuit 1401 for delaying an input signal applied to an input terminal 1400 by one line, a second delay circuit 1402 for delaying an output signal of the first delay circuit 1401 by one line further, a first multiplier 1403 for multiplying the input signal by -1/4, a second multiplier 1404 for halving the output signal of the first delay circuit 1401, a third multiplier 1405 for multiplying an output signal of the second delay circuit 1402 by -1/4, a first adder 1406 for summing the values obtained by the first to third multipliers, a band-pass filter 1407 for filtering the frequency band in the vicinity of the color subcarrier in the output signal of the first adder 1406, an one field delay circuit 1408 for delaying the output signal of the first delay circuit 1401 by one field, a second adder 1409 for adding the output signal of the first delay circuit 1401 and an output signal of the one field delay circuit 1408, a trap circuit 1410 for suppressing a signal having the frequency band in the color subcarrier in an output signal of the second adder 1409, a high-pass filter 1411 for filtering the high frequency component of an output signal of the trap circuit 1410, an absolute value circuit 1412 for obtaining the absolute value of an output signal of the high-pass filter 1411, an amplitude comparator 1413 for comparing an output signal of the absolute value circuit 1412 with a predetermined threshold TH, an amplitude limiter 1414 for limiting an output signal of the band-pass filter 1407 in response to an output value of the amplitude comparator 1413, a chrominance signal output terminal 1415 for outputting an output signal of the amplitude limiter 1414 as the chrominance signal, a subtractor 1416 for subtracting an output signal of the amplitude limiter 1414 from the output signal of the first delay circuit 1401, and a luminance signal output terminal 1417 for outputting an output signal of the subtractor 1416 as the luminance signal.
With the conventional luminance signal and chrominance signal separating apparatus as shown above, when an NTSC signal is inputted through the input terminal 1400, it is delayed through the first and second delay circuits, 1401 and 1402 and then subjected to multiplications respectively by the first to third multipliers 1403-1405 and summed up by the first adder 1406. This is of a comb filter structure to output the chrominance signal. This chrominance signal is sent to the band-pass filter 1407 to take out the signal having the frequency component in the vicinity of the color subcarrier.
The output signal of the first delay circuit 1401 is also inputted to the one field delay circuit 1408. The output signals of the first delay circuit 1401 and one field delay circuit 1408 are added up by the second adder 1409. The color subcarrier which is subjected to delay of one field is opposite in phase to that which is not subjected thereto, so that the carrier chrominance signal is canceled from the output signal of the second adder 1409.
The output signal of the second adder 1409 is subjected to suppression of the frequency component in the vicinity of the color subcarrier in the trap circuit 1410. The output signal of the trap circuit 1410 is inputted to the high-pass filter 1411 to output the high frequency component signal. The output signal of the high-pass filter 1411 is inputted to the absolute value circuit 1412 to take the absolute value thereof. The output signal of the absolute value circuit 1412 is compared with a predetermined reference value TH in the amplitude comparator 1413 to output the difference thereof. The amplitude limiter 1414 receives the output signal of the bond-pass filter 1407 and the output signal of the amplitude comparator 1413 to limit the amplitude of the signal received from the bond-pass filter 1407 in response to the signal received from the amplitude comparator 1413. The output signal of the amplitude limiter 1414 is subtracted from the output signal of the first delay circuit 1401 in the subtractor 1416 and outputted from the luminance signal output terminal 1417.
The operation for the input signal including a high frequency luminance signal will be explained below. In the case where the luminance signal has a high frequency component, the output signal of the high-pass filter 1411 also includes the high frequency component and the output value of the absolute value circuit 1412 is increased so as to be larger than the predetermined threshold TH in the amplitude comparator 1413. As a result, unnecessary components to be mixed into the chrominance signal are suppressed in the amplitude limiter 1414 to prevent the generation of cross color.
However, with the conventional luminance signal and chrominance signal separating apparatus of the second example as explained above, when a moving picture signal is inputted, in spite of the fact that there is no leakage entrance of high frequency components from the luminance signal into the chrominance signal, the second adder 1409 outputs a signal and as a result, a problem may arise in that the chrominance signal is suppressed despite the lack of mixture of the high frequency components of the luminance signal into the chrominance signal in the amplitude limiter 1414.